This invention relates to a high-temperature screw lubricating paste. More particularly, this invention relates to a homogeneous preparation of tin dioxide, or tin dioxide mixed with a thermally stable inorganic filler in a carrier oil.
Threaded connections are used in all kinds of industrial branches to firmly assemble the various types of structural members and to disassemble them easily when the occasion arises. In all cases of threaded connections being exposed to extreme stress by high temperatures or pressures, possibly also combined with a heavy stress due to corrosion, these threaded connections (screws and nuts) are made of high strength materials. Usually in such cases materials are selected from austenitic steels or nickel base alloys which, in addition to their special mechanical strength and high-temperature strength, have at the same time an excellent corrosion resistance. Threaded connections from materials such as these are used for instance for turbines, internal combustion engines, chemical industry fittings, gas producers and oil refinery facilities.
To prevent damages, as for instance seizure and galling, in threaded connections such as these, an effective lubrication, namely a possibly complete separation of the thread surfaces, is a must. To achieve this, high-temperature screw lubricating pastes are normally used which are based on the various finely divided inorganic solids being prepared in mineral oils and/or synthetic oils as carrier oils.
The high-temperature pastes commonly in use normally contain as solids graphite or other solid lubricants such as molybdenum disulfide and/or metal powders such as lead, zinc, copper or nickel. These solid-containing pastes are, however, not satisfactory in every respect, as they meet the following requirements only in part or insufficiently:
(a) Easy dismantling of the threaded connection after high-temperature operation, i.e. a break-away torque that is not too high.
(b) Coefficient of friction between approximately 0.10 and 0.14.mu. with as little variation as possible after repeated loosening and tightening.
(c) Temperature stability of the lubricating layer up to approximately 1200.degree. C.
(d) Good corrosion resistance.
(e) No reaction with the screw material.
Careful investigations revealed that the unsatisfactory behavior of the known solid-containing screw pastes during high-temperature operation is to a great extent likely to be due to the fact that the metallic (and possibly also halogen-, phosphor- or sulfur-containing) constituents of such pastes react with screw materials such as austenitic steels or nickel base alloys and lead to metallurgical changes resulting in cracking, flank fractures or even bolt fractures. Graphite leads to carburization of the screw material, forming chromium carbide with microstructural changes and intercrystalline brittleness. Lead melts, diffuses along the grain boundaries into the austenite and produces cracking and brittleness of solder. The same is true for zinc, too. The use of copper is disadvantageous in so far as at high temperatures it can lead to the so-called red shortness. Nickel powder is numbered among the carcinogenic substances. Halogen-, phosphor- and sulfur-containing components decompose at high temperatures, setting these elements free and leading to metallurgical changes and damages of the screw material.